it is not genetic, it is environmental. Children of parents with less intelligence will not be raised to be intelligent. They might be lucky/resilience and try to get the most support outside the house, but it is much harder to accomplish, and often is even met with harassment at home, due to the rest of the family being insecure about their own lack of intelligence. And that is only if they rebel, which is not necessarily true as they will not only lack easy access to basic knowledge about the world/science, but will also not be introduced to the importance of learning about it from their closest figures of authority. Escaping that cycle it is even harder if the family is facing economic hardship, which is true for most modern families in general. It really isn’t that hard to figure that out, the kneejerk reaction that the statement always gets is annoying.